1. Field of the Invention
The invention relates to a subscriber telephone line interface circuit, that is to say a circuit forming part of a telephone exchange and intended to be connected to an individual subscriber telephone line for transmitting signals over this line and receiving signals from this line.
2. Description of the Prior Art
The average rate of occupation of a telephone line is about 10%, which means that for about 90% of the time the interface circuit has nothing to do but observe the current on the line for detecting lifting of the handset. Such observation does not require a very high power consumption but, in fact, the other unused functions of the circuit remain active and wastefully consume power. It is essential to reduce this power as much as possible so as to reduce the space required by the telephone exchanges which comprise thousands of such circuits. The space required in fact depends mainly on the heat dissipation requirements of the power consumed in these numerous circuits.
In presently known telephone exchanges, the interface circuit connected to a telephone line is in fact formed of two integrated circuits, one providing an interface function properly speaking and being fed with a voltage of about 50 volts and the other providing a certain "intelligence", namely signal processing; the second circuit is fed with about 10 volts.
The high tension integrated circuit in particular imposes the voltage on the line and it measures the AC and DC current present on the line. The result of the measurement is transmitted to the low tension integrated circuit which processes this information and sends a feed-back signal to the high tension circuit. The low tension circuit further serves for transmitting signals over the line and more particularly conversation signals.
There exists then a permanent feed-back loop between the two circuits. This loop exists also when the handset is replaced.
To reduce the power consumed during the line observation phases (90% of the time), it has already been proposed to simply reduce the currents supplying the different parts of the circuits, since the operating requirements are not the same as outside line observation phases; in fact, during the observation phase there is no AC current to be transmitted.
Thus the overall power consumed may be reduced but only in a small proportion.